Russia has warned that the presence of foreign military personnel could "destabilize" the situation in Ukraine, after U.S. paratroopers arrived this week to train Ukrainian solders.

A spokesman for President Vladimir Putin, Dmitry Peskov, told reporters Friday that instructors from "third countries," besides Russia or Ukraine, could upset the situation in Ukraine's east, where pro-Russia rebels are engaged in an armed rebellion against Ukrainian forces.

A U.S. airborne brigade arrived in Ukraine this week to conduct a series of training sessions with Ukrainian troops.

On Thursday, President Putin accused the United States of trying to dominate international relations, but said Russia is still "always open to cooperation."

Russian President Vladimir Putin takes part in a live broadcast nationwide call-in in Moscow, April 16, 2015.

Referring to the United States, Putin said "major superpowers that claim their uniqueness and consider themselves the only center of power in the world" need "vassals," not "allies."

He said that after World War II Moscow tried to impose its model of development on many Eastern European countries by force, and "there was nothing good" in this. The Americans are now "acting in a similar way by trying to impose their model around the world," he said, adding that they too would fail.

The Russian president also suggested that the United States had pressured some European leaders not to attend events in Moscow on May 9 commemorating the 70th anniversary of the end of World War II in Europe.